There are many reasons why it may be desirable to separate a composite material into its constituent elements. Just as there are many such reasons, there are many ways or methods by which this can be accomplished. For one, it is well known that some composite or combination materials can be mechanically separated by means such as sieves, sorters and diverters. Further, it is known that chemical processes are often useful for separating composites into their separate parts. It happens, however, that some composite materials are extremely difficult to process and, therefore, do not readily lend themselves to the more conventional methods of processing. In particular, nuclear waste is such a composite material.
Recently, efforts have been made to process materials by first vaporizing them, and then causing the vaporized constituent elements to separate from each other. One such process involves the use of a plasma centrifuge. In a plasma centrifuge, the charged particles of a plasma are caused to rotate around a common axis, and to collide with each other as they rotate. As a consequence of these collisions, the heavier mass particles move farther away from the axis of rotation than do the lighter mass particles. Accordingly, the particles are separated according to their respective masses. More recently, however, plasma filters have been developed which rely on physical principles that are much different than those relied on by plasma centrifuges.
An example of a plasma filter and its methods of operation are provided in U.S. Pat. No. 6,096,220, issued to Ohkawa, for an invention entitled "Plasma Mass Filter" which is assigned to the same assignee as the present invention. Several aspects of a plasma filter that distinguish it from a plasma centrifuge are noteworthy. In particular, unlike a plasma centrifuge, it is important that a plasma filter operates with a plasma density that is below a collisional density. By definition, and as used herein, a collisional density occurs when the ratio of a cyclotron angular frequency to a collisional frequency is greater than one (i.e. .omega..sub.c /.nu.&gt;1). Stated differently, in a plasma having a density below its collisional density, there is a high probability that a charged particle will experience at least one orbited rotation before colliding with another charged particle in the plasma. Thus, very much unlike a plasma centrifuge, a plasma filter avoids collisions between the charged particles. Another aspect which distinguishes a plasma filter from a plasma centrifuge is that crossed electric and magnetic fields can be employed in a plasma filter to selectively confine the trajectories of orbiting charged particles. Specifically, as disclosed for the plasma mass filter by Ohkawa mentioned above, charged particles having a mass-charge ratio below a determinable cut-off mass, M.sub.c, will be confined within a space between the axis of rotation and a radial distance, "a," therefrom. As previously disclosed by Ohkawa, for a cylindrical plasma mass filter chamber, M.sub.c =ea.sup.2 B.sup.2 /(8V.sub.ctr) wherein there is a radius, "a," a uniform axial magnetic field, "B," and a parabolic radial voltage profile with a central voltage, "V.sub.ctr," with the wall of the cylinder grounded. The charge on the heavy ion to be separated is "e."
It can happen that it may be desirable, or necessary, to separate a composite material into more than two parts. For example, it may be desirable to separate a nuclear waste into three or more component parts. For example, one part may be a radioactive toxic nuclear component which must be disposed of under most careful circumstances. On the other hand, another part of the composite material may be useful in other different processes. Still another part may be disposable by more ordinary and conventional means.
In light of the above, it is an object of the present invention to provide a multi-mass filter that is capable of separating a multi-species plasma into more than two constituent parts. Another object of the present invention is to provide a multi-mass filter which effectively confines charged particles of different mass-charge ratios to trajectories that direct the charged particles into respectively different regions for segregated collection. Still another object of the present invention is to provide a multi-mass filter that is relatively simple to manufacture, is easy to use, and is comparatively cost effective.